The present invention relates to wrenches.
When nuts are to be tightened or loosened with fixed wrenches, it is important that deviations in the shape of the nuts do not cause damage to the nuts or to the wrenches. Two types of shape deviation are especially important to consider: corner damage and differences in the across-flat dimension. It is also important that the wrenches are able to be used for nuts which are coated with paint or rust-protective layers.
In order not to cause or worsen damage to the nuts, the force acting from the wrench on the nut should be spread over a sufficient surface so as not to damage surface coatings, and should not act too near the corners. The force should be relatively close to normal to the surface, since frictional shear forces can easily damage coatings. The force value for a given torque depends on the leverage, measured as the distance from the line of the force to the center line of the nut. If the force becomes excessive there is also an elastic deformation of the wrench which makes the angular motion indistinct and may cause large stresses; in severe cases the wrench may break.
In the simplest types of wrenches the jaws or contact surfaces are flat. This means that undersize nuts are contacted only at the corners, which are easily damaged. On nuts where the corners are already damaged, the forces will then act closer to the center of the sides and become so large that the wrench may be damaged and the angular motion indistinct.
In patents U.S. Pat. Nos. 3,242,775 and 4,930,378 it has been suggested that the jaws of the wrench be made convex with large radius to spread the force over a larger area, but for such wrenches the contact point will vary very much with the across-flat dimension, and oversize nuts will overstress the wrench. If the wrench is made as in U.S. Pat. No. 5,481,948 with jaws which are flat with convexly rounded ends, the forces will be acceptable and the contact points relatively independent of the across-flat dimension, but for nuts with damaged corners, the forces will be excessive and likely to worsen the damage. If the jaws are made with fully serrated surfaces as in U.S. Pat. No. 4,126,063 the forces on nuts with corner damage will be limited since frictional shear forces are utilized, but this may hurt painted or galvanized nuts. It is also known to make jaws which are serrated on the inner half and flat on the outer half, with the purpose of pulling the nut into the gap between the jaws.